Coal in its virgin state, particularly low rank coal, is sometimes treated to improve its usefulness and thermal energy content in a process known as beneficiation. The beneficiation treatment can include drying the coal and subjecting the coal to a pyrolysis process to drive off low boiling point organic compounds and heavier organic compounds. This thermal treatment of coal, also known as low temperature coal carbonization, causes the release of certain volatile hydrocarbon compounds having value for further refinement into liquid or gaseous fuels and other coal-derived liquids (CDLs) and chemicals. These volatile components can be removed from the effluent or gases exiting the pyrolysis process. The pyrolytic treatment of coal also leaves behind a product known as coal char, composed of carbon and non-volatile minerals, elements and ash.
Coal char, unless passivated, is highly susceptible to self combustion and cannot be stored easily. Thus, various processes have been developed to passivate coal char. One such pyrolysis and passivation process is disclosed in U.S. Pat. No. 5,601,692 to Rinker et al. Rinker et al. describe how coal is heated to as much as 590° C. during pyrolysis to drive off low end volatile components, after which the char readily reabsorbs moisture and oxygen from an ambient atmosphere in an exothermic process that is pyrophoric, or has the tendency to self ignite. To passivate the char particles, Rinker et al. rapidly decreases the temperature by 100° C. or more to a temperature of about 150° C. to about 200° C., by any conventional means such as spraying with water. The relatively large char particles are then conveyed to an oxidative passivation unit where the particles are contacted with an oxygen-containing process gas in a cross flow system that is isolated from ambient air. The process gas may contain from 3% to about 23% oxygen (by weight) depending inversely on its temperature. The process gas intermixes with the char particles producing two results: (a) extracting heat to cool the char particles further; and (b) producing a chemisorption of some of the oxygen onto or into the char particles. This chemisorption and rehydration reduces the propensity of the char to self-ignite, thereby rendering it suitable for storage, transport or further processing.
While the current passivation processes are valuable, they are time consuming and the oxidative chemisorptions step may need to be repeated once or more at lower temperatures to effect complete passivation. Furthermore, the chemisorption process itself is exothermic, so even if the char is cooled prior to passivation, additional heat is generated in the passivation process and must also be removed. Prior art methods to address this exothermic heat removal have generally involved the flow of large quantities of the recycle gas to limit the temperature increase. However, this become impractical when the heat removal demands of a rapid, continuous flow process are considered.
It would be advantageous if passivation processes could be improved to produce passivated char more quickly and/or using less energy. It would also be advantageous if passivation processes could be improved to permit more control over the factors of the passivation, such as the level of oxygen input to the system, the rate of moisture addition, or the rate and location at which heat is extracted.